1. Field of the Invention
The present invention relates to a hydraulic circuit for a backhoe implement of a work vehicle, and more particularly, to a hydraulic circuit of the above type having a plurality of pumps driven by a common engine and a plurality of actuators.
2. Description of the Related Art
A hydraulic circuit of the above-described type is known from a Japanese laid-open utility model gazette No. 62-31166, for example. This circuit includes first through third pumps respectively connected to corresponding actuators, and all these three pumps are driven by a single engine. For this reason, magnitude of the engine output is so designed as to avoid an engine stop even if a sum pressure of oil fed from all the pumps reaches a relief pressure. During a vehicle run or a work a such as, excavating work, the engine receives a large load for the first and second pumps, while the third pump for e.g. swivelling remains idle, that is unloaded. Accordingly, the actual total load affecting the engine is smaller than the above-described, predetermined tolerable load, which means that part of the engine output remains unused during the vehicle run or the work. This is undesirable with view to the maximum efficient use of the engine output.
One conceivable method to improve the efficiency is to provide a manually operable, relief pressure switchover means for switching the relief pressure between a high state and a low state to be fed to a first feed oil passage connected to the first pump and a second feed oil passage connected to the second pump. However, whether the relief pressure switchover means can function properly or not depends on whether the user operates the same properly or not. This can be difficult when the user's attention is diverted for carrying out various tasks in a short time period. With the pressure being erroneously set to low, the user will fail to utilize the engine output fully for the desired vehicle run or the work, just as the case having no such relief pressure switchover means at all. In the opposite case, the user will suffer frequent engine stops.
The object of the invention becomes now clear. The invention intends to provide a hydraulic circuit which permits full use of the engine output all the time and which permits a proper switching of the relief pressure in an automatic fashion.